In a statement on his website, Ryan wrote: "It was Dr. Murphy's decision to move on to the next chapter of his life, and I support it."

Gov. Tom Wolf will now decide when 18th Congressional District voters will choose Murphy's replacement. Under state law, the governor must issue a writ of election within ten days of the seat being vacated, which specifies when a special election will be held no sooner than 60 days from the date the writ is issued. With Murphy's resignation effective Oct. 21, the earliest the special election could be held is later December or early January.

The governor has not signaled his intentions, but the law laying out special election procedures would allow him to schedule the contest to coincide with next year's statewide primary elections in May.

Republican state lawmakers are already eyeing a run for Murphy’s seat.

State Sen. Guy Reschenthaler said Thursday that he's running, though he made the announcement before the news that Murphy would resign rather than serve out his term. Reschenthaler was elected to the state senate in 2015 after serving as a prosecutor in the U.S. Navy Judge Advocate General's Corps.

State Sen. Kim Ward has also said she will run for the seat. Ward has represented parts of Westmoreland County since 2009 and currently serves as Vice Chair of the Senate Appropriations Committee.

Additionally, fourth-term Rep. Rick Saccone has said he's considering running. The retired Air Force captain announced his intention to run for U.S. Senate in February.

Three Democrats have filed to run for the seat: Mike Crossey, a retired teacher and former Allegheny County Councilman; Pam Iovino, a Navy veteran and Assistant Secretary for Congressional Affairs with the U.S. Department of Veterans Affairs under George W. Bush; and Bob Solomon, an emergency medicine physician.

The district has been a safe Republican seat, with Republican Donald Trump beating Democrat Hillary Clinton by a margin of three-to-two in last November's presidential election.

*This post was updated to include information about a special election to fill Murphy's vacant House seat, as well as the fact that Sen. Kim Ward has declared her candidacy and th names of the three Democrats who have filed to run for the seat in the May primary.